Ceramic igniters

ABSTRACT

New methods are provided or manufacture ceramic resistive igniter elements that include extrusion of one or more layers of the formed element. Ceramic igniters also are provided that are obtainable from fabrication methods of the invention.

The present application claims the benefit of U.S. provisional application No. 60/623,478 filed Oct. 28, 2004, which is incorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

In one aspect, the invention provides new methods for manufacture ceramic resistive igniter elements that include extrusion of one or more layers of the formed element. Igniter elements also are provided obtainable from fabrication methods of the invention are provided.

2. Background

Ceramic materials have enjoyed great success as igniters in e.g. gas-fired furnaces, stoves and clothes dryers. Ceramic igniter production includes constructing an electrical circuit through a ceramic component a portion of which is highly resistive and rises in temperature when electrified by a wire lead. See, for instance, U.S. Pat. Nos. 6,582,629; 6,278,087; 6,028,292; 5,801,361; 5,786,565; 5,405,237; and 5,191,508.

Typical igniters have been generally rectangular-shaped elements with a highly resistive “hot zone” at the igniter tip with one or more conductive “cold zones” providing to the hot zone from the opposing igniter end. One currently available igniter, the Mini-Igniter™, available from Norton Igniter Products of Milford, N.H., is designed for 12 volt through 120 volt applications and has a composition comprising aluminum nitride (“AlN”), molybdenum disilicide (“MoSi₂”), and silicon carbide (“SiC”).

Igniter fabrication methods have included batch-type processing where a die is loaded with ceramic compositions of at least two different resistivities. The formed green element is then densified (sintered) at elevated temperature and pressure. See the above-mentioned patents. See also U.S. Pat. No. 6,184,497.

While such fabrication methods can be effective to produce ceramic igniters, batch-type processing presents inherent limitations with respect to output and cost efficiencies.

Current ceramic igniters also have suffered from breakage during use, particularly in environments where impacts may be sustained such as igniters used for gas cooktops and the like.

It thus would be desirable to have new ignition systems. It would be particularly desirable to have new methods for producing ceramic resistive elements. It also would be desirable to have new igniters that have good mechanical integrity.

SUMMARY OF THE INVENTION

We now provide new methods for producing ceramic igniter elements which includes extruding ceramic material to thereby form the ceramic element. Such extrusion fabrication can provide enhanced output and cost efficiencies relative to prior approaches such as die cast methods as well as provide igniters of notable mechanical strength.

More particularly, preferred methods of the invention include extruding one or more layers to form a ceramic element. If multiple layers are extruded, preferably those layers have differing resistivities to provide regions of distinct conductivity in the formed element. For example, an element may be formed by co-extrusion of one or more multiple, sequential layers of 1) an optional insulator (heat sink); 2) conductive zone; 3) resistive hot zone; and 4) second conductive zone. The second conductive zone may be applied for only a portion of the igniter to provide an exposed resistive hot zone for fuel ignition.

Preferred methods of the invention also include formation of multiple igniter elements in a single process which includes a step of extruding ceramic material.

In one aspect of such preferred methods, a plurality of operational igniter elements can be produced from one or more billet or tile elements where such billet or tile elements are produced by extruding ceramic material. For example, one or more ceramic tile elements can be produced by extrusion or co-extrusion of ceramic material. Thereafter, the tile elements can be thermally treated to remove any binders or other carriers used in the extrusion process and optionally densified (such as at elevated pressures and temperatures), and then the densified tile element(s) cut to provide igniter-shaped elements of desired dimensions. Such steps also may be conducted in alternate sequence, e.g. prior to densification, the tile elements(s) may be cut to form igniter-shaped elements of desired dimensions and the thus produced green state igniter elements may then be optionally densified at elevated pressures and temperatures.

Fabrication methods of the invention may include additional processes for addition of ceramic material to produce the formed ceramic element. For instance, one or more ceramic layers may be applied to a formed element such as by dip coating, spray coating and the like of a ceramic composition slurry.

Preferred ceramic elements obtainable by methods of the invention comprise a first conductive zone, a resistive hot zone, and a second conductive zone, all in electrical sequence. In certain preferred embodiments, the first conductive zone will be positioned within an inner area of the igniter element and encased or enveloped at least in part by the second, outer positioned conductive zone, as further discussed below. Preferably, during use of the device electrical power can be applied to the first or the second conductive zones through use of an electrical lead (but typically not both conductive zones). For at least some preferred applications, at least a substantial portion of the first conductive zone does not contact a ceramic insulator (heat sink). Such absence of a ceramic insulator region can provide enhanced time-to-ignition temperature performance of the igniter.

Particularly preferred igniters of the invention of the invention will have a rounded cross-sectional shape along at least a portion of the igniter length (e.g., the length extending from where an electrical lead is affixed to the igniter to a resistive hot zone). More particularly, preferred igniters may have a substantially oval, circular or other rounded cross-sectional shape for at least a portion of the igniter length, e.g. at least about 10 percent, 40 percent, 60 percent, 80 percent, 90 percent of the igniter length, or the entire igniter length. A substantially circular cross-sectional shape that provides a rod-shaped igniter element is particularly preferred. Such rod configurations offer higher Section Moduli and hence can enhance the mechanical integrity of the igniter.

Igniters of the invention may have a variety of configurations. In a preferred configuration, a conductive shaft element is positioned within a conductive tube element and both the shaft and tube elements mate with a hot zone cap or end region.

Ceramic igniters of the invention can be employed at a wide variety of nominal voltages, including nominal voltages of 6, 8, 10, 12, 24, 120, 220, 230 and 240 volts.

The igniters of the invention are useful for ignition in a variety of devices and heating systems. More particularly, heating systems are provided that comprise a sintered ceramic igniter element as described herein. Specific heating systems include gas cooking units, heating units for commercial and residential buildings, including water heaters.

Other aspects of the invention are disclosed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (includes FIGS. 1A through 1C) depicts a preferred production method of the invention;

FIG. 2 shows a cut-away view along line 1-1 of FIG. 1C;

FIG. 3 (includes FIGS. 3A through 3D) depicts a further preferred production method of the invention;

FIGS. 4 shows a further preferred igniter of the invention; and

FIGS. 5A and 5B shows a further preferred igniter of the invention; FIG. 5B is a view taken along line 5B-5B of FIG. 5A;

FIGS. 6A and 6B shows a further preferred igniter of the invention; FIG. 6B is a view taken along line 6B-6B of FIG. 6A; and

FIGS. 7 and 8 show further preferred igniter and fabrication methods; and

FIGS. 9A, 9B and 9C an additional preferred igniter and fabrication method.

DETAILED DESCRIPTION OF THE INVENTION

As discussed above, new methods are now provided for producing ceramic igniter elements that include extrusion of one or more layers of the element.

As typically referred to herein, the term extrusion, extruding or other similar term indicates the general process where a material is forced through or otherwise advanced through a shape-inducing member such as a die element, where the die may be suitably formed of e.g. a polymer, metal, combinations thereof, etc. In extrusion formation of igniter elements of the invention, a ceramic material (such as a ceramic powder mixture, dispersion or other formulation) or a pre-ceramic material or composition may be advanced through a shape-inducing element. Suitably, the extruded material may be cured or otherwise hardened after exiting the shape-inducing element.

Referring now to the drawings, FIGS. 1A through 1C show a preferred fabrication method of the invention. As shown in FIG. 1A, an igniter element 10 is produced by co-extrusion of multiple layers that have differing resistivities. In a preferred system, the inner layer is a conductive layer 12, an intermediate layer is a more resistive hot zone layer 14, and the exposed outer layer is a second conductive layer 16.

Extrusion of the igniter elements may be suitably conducted by forming a fluid formulation of a ceramic composition and advancing the ceramic formulation through a die element that provides the igniter of desired configuration.

For instance, a slurry or paste-like composition of ceramic powders may be prepared, such as a paste provided by admixing one or more ceramic powders with an aqueous solution or an aqueous solution that contains one or more miscible organic solvents such as alcohols and the like. A preferred ceramic slurry composition for extrusion may be prepared by admixing one or more ceramic powders such as MoSi₂, SiC, Al₂O₃, and/or AlN in a fluid composition of water optionally together with one or more organic solvents such as one or more aqueous-miscible organic solvents such as a cellulose ether solvent, an alcohol, and the like. The ceramic slurry also may contain other materials e.g. one or more organic plasticizer compounds optionally together with one or more polymeric binders.

A wide variety of shape-forming or inducing elements may be employed to form an igniter element, with the element of a configuration corresponding to desired shape of the extruded igniter. For instance, to form a rod-shaped element, a ceramic powder paste may be extruded through a cylindrical die element. To form a stilt-like or rectangular-shaped igniter element, a rectangular die may be employed.

After extrusion, the shaped igniter suitably may be dried e.g. in excess of 50° C. or 60° C. for a time sufficient to remove any solvent (aqueous and/or organic) carrier.

The examples which follow describe preferred extrusion processes to form an igniter element.

As shown in FIG. 1B, a portion of conductive layer 16 can be removed to expose the resistive hot zone 14. The exposed hot zone length (shown as length “a” in FIG. 1B) can be varied to provide optimal performance for a targeted voltage.

The igniter element 10 then may be further processed as desired. For example, as shown in FIG. 1C, igniter 10 may be core-drilled to provide inner void region 18. The formed igniter 10 also may be further densified such as under conditions that include temperature and pressure.

A suitable igniter electrical path can be seen in FIG. 2 where electrical power enters the igniter system 10 through the interposed conductive core element 12 that mates with resistive hot zone 14. Proximal end 12 a of conductive element 12 and 10 a of conductive element 10 may be affixed such as through brazing to an electrical lead (not shown) that supplies power to the igniter during use. The igniter proximal end 10 a suitably may be mounted within a variety of fixtures, such as where a ceramoplastic sealant material encases conductive element proximal end 12 a as disclosed in U.S. Published Patent Application 2003/0080103. Metallic fixtures also maybe suitably employed to encase the igniter proximal end.

As shown in FIG. 2, the igniter's 10 depicted electrical path extends from conductive core element 12 through resistive hot zone 14 then through outer, encasing conductive region 16. The igniter also can be configured whereby the electrical path runs in the opposite direction and extends conductive region 16 through resistive hot zone 14 and then through the conductive core element 12.

As can be seen in FIGS. 1C and 2, the first, inner conductive zone 12 is segregated through void region 18 from the other igniter areas until mating with hot zone 14 at the conductive zone distal portion 12 c. Further, as discussed above, in preferred systems such as those depicted in FIGS. 1C and 2, the proximal portion 12 a of the first conductive zone does not contact a ceramic heat sink (insulator) area that has been employed in certain prior systems. For at least many applications, suitably the igniter may not contain any insulator or heat sink region and will contain only two regions of the differing resistivity, i.e. the igniter will contain only conductive (cold) zone(s) and a higher resistivity (hot) zone.

Such absence of a ceramic insulator from at least a substantial portion of the first conductive zone length can provide significant advantages, including enhanced time-to-temperature performance of the igniter. As referred to herein, “a substantial portion of the first conductive zone length” indicates that at least about 40 percent of the length of the conductive zone as measured from the point of affixation of an electrical lead to the mating hot zone (as shown by distance b is FIG. 2) does not contact a ceramic insulator material. More preferably, at least about 50, 60, 70, 80, 90 or 95 percent or the entire length of the conductive zone as measured from the point of affixation of an electrical lead to the mating hot zone (as shown by distance b is FIG. 2) does not contact a ceramic insulator material. In particularly preferred systems, at least a substantial portion of the first conductive zone length is exposed such as to void area 18 as generally depicted in the igniters exemplified in FIGS. 1C and 2.

As referred to herein, the term “time-to-temperature” or similar term refers to the time for an igniter hot zone to rise from room temperature (ca. 25° C.) to a fuel (e.g. gas) ignition temperature of about 1000° C. A time-to-temperature value for a particular igniter is suitably determined using a two-color infrared pyrometer. Particularly preferred igniters of the invention may exhibit time-to-temperature values of about 3 seconds or less, or even about 2 seconds or less.

As discussed above, and exemplified in FIG. 1, for at least certain preferred systems, at least a substantial portion of the igniter length has a rounded cross-sectional shape along at least a portion of the igniter length, such as length b shown in FIG. 2. FIG. 1 depicts a particularly preferred configuration where igniter 10 has a substantially circular cross-sectional shape for about the entire length of the igniter to provide a rod-shaped igniter element. However, preferred systems also include those where only a portion of the igniter has a rounded cross-sectional shape, such as where up to about 10, 20, 30, 40, 50, 60, 70 80 or 90 of the igniter length (as exemplified by igniter length b in FIG. 2) has a rounded cross-sectional shape; in such designs, the balance of the igniter length may have a profile with exterior edges.

FIG. 3A through 3D depict a further preferred igniter fabrication method of the invention where igniter element 20 is formed by extrusion of a conductive core region 22 that is encased within an insulator (heat sink) inner layer 24.

As shown in FIG. 3B, a second outer conductive layer 26 can be applied to the extruded igniter element followed by application of a resistive hot zone region 28, as shown in FIG. 3C. As discussed above, ceramic layers 26 and 28 may be applied by any of a number of methods. A preferred application method is dip coating of igniter element in a ceramic composition slurry with appropriate masking of non-coated igniter regions.

For such dip coating applications, a slurry or other fluid-like composition of the ceramic composition may be suitably employed. The slurry may comprise water and/or polar organic solvent carriers such as alcohols and the like and one or more additives to facilitate the formation of a uniform layer of the applied ceramic composition. For instance, the slurry composition may comprise one or more organic emulsifiers, plasticizers, and dispersants. Those binder materials may be suitably removed thermally during subsequent densification of the igniter element.

Dip coating may be conducted by immersion of the igniter element in the ceramic composition slurry. Preferred dip coating processes and ceramic composition slurries for dip coating are exemplified in the examples which follow.

As shown in FIG. 3D, ceramic insulator region 24 may be at least partially removed such as by drilling to provide void regions 30. Thus, as depicted in FIG. 3D, interposed first conductive zone 22 extends from a proximal end 22 a (which may have an affixed electrical lead as discussed above) and extends to resistive zone 28 that mates with second conductive zone 26, positioned above partially removed insulator layer 24 and interposed void region 30.

FIG. 4 shows a further preferred igniter 40 of the invention that can be provided by co-extrusion of an insulator ceramic core with an encasing resistive zone 42 (such as resistive zone 28 shown in FIG. 3C). Conductive zone 44 then may be applied such as by dip coating the igniter element in a slurry of the conductive ceramic composition. Flats 46 may be formed on igniter faces as generally shown in FIG. 4 either by machining after densification or in the green state. As discussed above with respect to igniter 20 in FIG. 3D, ceramic insulator region may be at least partially removed such as by drilling to provide void region 48, or more preferably at least partial removal of the insulator region in the final igniter element may be provided by coextruding a hollow tube element.

Significantly, methods of the invention can facilitate fabrication of igniters of a variety of configurations as may be desired for a particular application. To provide a particular configuration, an appropriate shape-inducing die is employed through which a ceramic composition (such as a ceramic paste) may be extruded.

For instance, a die with a substantially square profile may be employed to produce the igniter element 50 depicted in FIGS. 5A and 5B which comprises a rectangular-like or a silt-like core conductive zone 52 with angular cross-sectional shape (more particularly, substantially square cross-sectional shape as clearly depicted in FIG. 5B) and similarly angular outer conductive zone 54 and hot zone (hot zone not shown in cut-away view of FIG. 5A).

A die with an irregular rounded shaped profile may be employed to form an element 60 as shown in FIGS. 6A and 6B with core conductive zone 62 and outer conductive zone 64 each having irregular rounded cross-sectional shapes.

Dimensions of igniters of the invention may vary widely and may be selected based on intended use of the igniter. For instance, the length of a preferred igniter (length b in FIG. 2) suitably may be from about 0.5 to about 5 cm, more preferably from about 1 about 3 cm, and the igniter cross-sectional width may suitably be from about (length c in FIG. 2) suitably may be from about 0.2 to about 3 cm.

Similarly, the lengths of the conductive and hot zone regions also may suitably vary. Preferably, the length of a first conductive zone (length d in FIG. 2) of an igniter of the configuration depicted in FIG. 2 may be from 0.2 cm to 2, 3, 4, or 5 more cm. More typical lengths of the first conductive zone will be from about 0.5 to about 5 cm. The height of a hot zone (length e in FIG. 2) may be from about 0.1 to about 2 cm, with a total hot zone electrical path length (length f in FIG. 2) of about 0.2 to 5 or more cm, with a total resistive zone path length (shown as the dashed line in FIG. 2) of about 0.5 to 3.5 cm generally preferred.

In preferred systems, the hot or resistive zone of an igniter of the invention will heat to a maximum temperature of less than about 1450° C. at nominal voltage; and a maximum temperature of less than about 1550° C. at high-end line voltages that are about 110 percent of nominal voltage; and a maximum temperature of less than about 1350° C. at low-end line voltages that are about 85 percent of nominal voltage.

As discussed above, preferred methods of the invention also include formation of multiple igniter elements in a single general process which includes a step of extruding ceramic material.

For example, as generally illustrated in FIG. 7, a ceramic tile element 70 can be provided by extruding ceramic material though a corresponding die element, with the tile element preferably formed through co-extrusion of multiple layers or regions of the tile with each layer or region having differing electrical resistivity. Thus, as exemplified in FIG. 7, multiple regions of the tile element are co-extruded to provide conductive region 72 and comparatively more resistive “hot” or ignition region 74.

The arrow shown in FIG. 7 depicts the direction of co-extrusion of the tile element. The ceramic tile element 70 may have a variety of dimensions and suitably may be sliced to provide 5, 10, 15, 20, 30, 40, 50, 80, 100 or more discrete igniter elements. For certain embodiments, the tile element suitably may have a thickness (dimension “a” in FIG. 7) that is the same thickness as the igniter elements formed after slicing of the tile. In other embodiments, such as discussed below with respect to FIGS. 9A through 9C where multiple tiles are aggregated, the tile element thickness may be only a portion of the thickness of the subsequently formed igniter elements.

Following extrusion, binder(s) and other organics present in the extruded ceramic material may be removed such as by thermal treatment. Thereafter, igniter elements may be formed in desired dimensions such as by slicing igniter element 70 in a direction perpendicular to the direction of extrusion (that extrusion direction shown by the depicted arrow in FIG. 7). As discussed above, the igniter elements may be densified if desired at elevated pressures and temperatures. Also, if desired, the igniter element may be further processed as desired, e.g. where an internal area is removed to form a so-called “slotted” igniter design around which slot an electrical path is provided.

FIG. 8 illustrates another preferred method where tile element 90 is provided through extrusion of a ceramic material though a corresponding die element, with the tile element preferably formed through co-extrusion of multiple layers or regions of the tile with each layer or region having differing electrical resistivity. Thus, as exemplified in FIG. 8, multiple regions of the tile element are co-extruded to provide conductive areas 86 and comparatively more resistive “hot” or ignition areas 88.

The arrow shown in FIG. 8 depicts the direction of co-extrusion of the tile element. The phantom line 82 depicts where the formed tile element 80 can be cut to two separate components. Thereafter, each tile component can be further sliced (such as perpendicular to depicted arrow) to provide individual igniter elements. In the embodiment depicted in FIG. 8, the formed igniter element includes an internal heat sink or insulator region 84 (rather than a slotted configuration discussed above with respect to FIG. 7). Either before or after slicing of the tile element the ceramic material may be densified such as at elevated pressures and temperatures.

FIGS. 9A through 9C illustrate yet another preferred fabrication process where multiple tile or billet elements are produced and then aggregated to form a plurality of igniter elements. More particularly, as generally illustrated in FIG. 9A, ceramic tile or billet element 90 is formed by co-extrusion of ceramic material through a corresponding die and includes a conductive region 92 and comparatively more resistive “hot” or ignition region 94. As shown in FIG. 9B, a separate ceramic tile or billet element 96 is formed by co-extrusion of ceramic material through a corresponding die and includes a insulator or heat sink region 98. The arrows shown in FIGS. 9A and 9B depict the direction of co-extrusion of the tile element. Binder(s) and other organics of the extruded ceramic materials can be removed by thermal treatment of the formed tile elements.

Then multiple tile elements may be aggregated, such as assembly of a three-tile stack of an element 96 between two elements 90 as shown in FIG. 9 c. That assembly 100 then be cut to provide individual igniter elements. As discussed above, the igniter elements may be densified if desired at elevated pressures and temperatures.

A variety of compositions may be employed to form an igniter of the invention. Generally preferred hot zone compositions comprise two or more components of 1) conductive material; 2) semiconductive material; and 3) insulating material. Conductive (cold) and insulative (heat sink) regions may be comprised of the same components, but with the components present in differing proportions. Typical conductive materials include e.g. molybdenum disilicide, tungsten disilicide, nitrides such as titanium nitride, and carbides such as titanium carbide. Typical semiconductors include carbides such as silicon carbide (doped and undoped) and boron carbide. Typical insulating materials include metal oxides such as alumina or a nitride such as AlN and/or Si₃N₄.

As referred to herein, the term electrically insulating material indicates a material having a room temperature resistivity of at least about 10¹⁰ ohms-cm. The electrically insulating material component of igniters of the invention may be comprised solely or primarily of one or more metal nitrides and/or metal oxides, or alternatively, the insulating component may contain materials in addition to the metal oxide(s) or metal nitride(s). For instance, the insulating material component may additionally contain a nitride such as aluminum nitride (AlN), silicon nitride, or boron nitride; a rare earth oxide (e.g. yttria); or a rare earth oxynitride. A preferred added material of the insulating component is aluminum nitride (AlN).

As referred to herein, a semiconductor ceramic (or “semiconductor”) is a ceramic having a room temperature resistivity of between about 10 and 10⁸ ohm-cm. If the semiconductive component is present as more than about 45 v/o of a hot zone composition (when the conductive ceramic is in the range of about 6-10 v/o), the resultant composition becomes too conductive for high voltage applications (due to lack of insulator). Conversely, if the semiconductor material is present as less than about 10 v/o (when the conductive ceramic is in the range of about 6-10 v/o), the resultant composition becomes too resistive (due to too much insulator). Again, at higher levels of conductor, more resistive mixes of the insulator and semiconductor fractions are needed to achieve the desired voltage. Typically, the semiconductor is a carbide from the group consisting of silicon carbide (doped and undoped), and boron carbide. Silicon carbide is generally preferred.

As referred to herein, a conductive material is one which has a room temperature resistivity of less than about 10⁻² ohm-cm. If the conductive component is present in an amount of more than 35 v/o of the hot zone composition, the resultant ceramic of the hot zone composition, the resultant ceramic can become too conductive. Typically, the conductor is selected from the group consisting of molybdenum disilicide, tungsten disilicide, and nitrides such as titanium nitride, and carbides such as titanium carbide. Molybdenum disilicide is generally preferred.

In general, preferred hot (resistive) zone compositions include (a) between about 50 and about 80 v/o of an electrically insulating material having a resistivity of at least about 10¹⁰ ohm-cm; (b) between about 0 (where no semiconductor material employed) and about 45 v/o of a semiconductive material having a resistivity of between about 10 and about 10⁸ ohm-cm; and (c) between about 5 and about 35 v/o of a metallic conductor having a resistivity of less than about 10⁻² ohm-cm. Preferably, the hot zone comprises 50-70 v/o electrically insulating ceramic, 10-45 v/o of the semiconductive ceramic, and 6-16 v/o of the conductive material. A specifically preferred hot zone composition for use in igniters of the invention contains 10 v/o MoSi₂, 20 v/o SiC and balance AlN or Al₂O₃.

As discussed, igniters of the invention contain a relatively low resistivity cold zone region in electrical connection with the hot (resistive) zone and which allows for attachment of wire leads to the igniter. Preferred cold zone regions include those that are comprised of e.g. AlN and/or Al₂O₃ or other insulating material; SiC or other semiconductor material; and MoSi₂ or other conductive material. However, cold zone regions will have a significantly higher percentage of the conductive and semiconductive materials (e.g., SiC and MoSi₂) than the hot zone. A preferred cold zone composition comprises about 15 to 65 v/o aluminum oxide, aluminum nitride or other insulator material; and about 20 to 70 v/o MoSi₂ and SiC or other conductive and semiconductive material in a volume ratio of from about 1:1 to about 1:3. For many applications, more preferably, the cold zone comprises about 15 to 50 v/o AlN and/or Al₂O₃, 15 to 30 v/o SiC and 30 to 70 v/o MoSi₂. For ease of manufacture, preferably the cold zone composition is formed of the same materials as the hot zone composition, with the relative amounts of semiconductive and conductive materials being greater.

A specifically preferred cold zone composition for use in igniters of the invention contains 20 to 35 v/o MoSi₂, 45 to 60 v/o SiC and balance either AlN and/or Al₂O₃.

At least certain applications, igniters of the invention may suitably comprise a non-conductive (insulator or heat sink) region, although particularly preferred igniters of the invention do not have a ceramic insulator insular that contacts at least a substantial portion of the length of a first conductive zone, as discussed above.

If employed, such a heat sink zone may mate with a conductive zone or a hot zone, or both. Preferably, a sintered insulator region has a resistivity of at least about 10¹⁴ ohm-cm at room temperature and a resistivity of at least 10⁴ ohm-cm at operational temperatures and has a strength of at least 150 MPa. Preferably, an insulator region has a resistivity at operational (ignition) temperatures that is at least 2 orders of magnitude greater than the resistivity of the hot zone region. Suitable insulator compositions comprise at least about 90 v/o of one or more aluminum nitride, alumina and boron nitride. A specifically preferred insulator composition of an igniter of the invention consists of 60 v/o AlN; 10 v/o Al₂O₃; and balance SiC. Another preferred heat composition for use with an igniter of the invention contains 80 v/o AlN and 20 v/o SiC.

The igniters of the present invention may be used in many applications, including gas phase fuel ignition applications such as furnaces and cooking appliances, baseboard heaters, boilers, and stove tops. In particular, an igniter of the invention may be used as an ignition source for stop top gas burners as well as gas furnaces.

Igniters of the invention also are particularly suitable for use for ignition where liquid fuels (e.g. kerosene, gasoline) are evaporated and ignited, e.g. in vehicle (e.g. car) heaters that provide advance heating of the vehicle.

Preferred igniters of the invention are distinct from heating elements known as glow plugs. Among other things, frequently employed glow plugs often heat to relatively lower temperatures e.g. a maximum temperature of about 800° C., 900° C. or 1000° C. and thereby heat a volume of air rather than provide direct ignition of fuel, whereas preferred igniters of the invention can provide maximum higher temperatures such as at least about 1200° C., 1300° C. or 1400° C. to provide direct ignition of fuel. Preferred igniters of the invention also need not include gas-tight sealing around the element or at least a portion thereof to provide a gas combustion chamber, as typically employed with a glow plug system. Still further, many preferred igniters of the invention are useful at relatively high line voltages, e.g. a line voltage in excess of 24 volts, such as 60 volts or more or 120 volts or more including 220, 230 and 240 volts, whereas glow plugs are typically employed only at voltages of from 12 to 24 volts.

The following non-limiting examples are illustrative of the invention. All documents mentioned herein are incorporated herein by reference in their entirety.

EXAMPLE 1 Igniter Fabrication

Powders of a resistive composition (15 vol % MoSi₂, 20 vol % SiC, remainder Al₂O₃) and an insulating composition (20 vol % SiC and 80 vol % Al₂O₃) were mixed with about 16 wt % water and 5 wt % Methyl Cellulose (Dow A4M) to form two pastes. The two pastes were loaded into the barrel of a piston extruder with the insulating paste forming a cylindrical core and the conducting paste forming a cylindrical sheath. The two mixes were co-extruded to form a coaxially clad rod of about 0.300″ diameter. The rod was then cured at 65° C. to remove the moisture and cut to 1-3″ lengths. The samples were dip-coated to coat one-half of the length with a slurry of conducting composition (30 vol % MoSi₂, 20 vol % SiC, remainder Al₂O₃). The slurry contains dispersants and a low viscosity base fluid containing isopropyl alcohol, PEG 400 (emulsifier; reaction product of stearic acid), SANTICIZER 160 (plasticizer; butyl benzyl), BUTWAR B76 (Monsanto; polyvinyl butyral), 111M dispersant (DARVAN). The coated sample was pre-sintered in Argon atmosphere at 1200° C. to bum out the binders, coated with boron nitride and densified at 1750° C. for 1 hour under a glass-hot isostatic press. The densified parts were cleaned by grit-blasting and an electrical circuit was formed by cutting grooves on opposite sides of the curved surfaces. The grooves were ⅛″ to ¼′ short of the part length at the uncoated end. The two faces of the coated end now separated by the groove form the two legs of the igniter and when connected to a power supply at a voltage of 60 volts attained a temperature of about 1200° C.

EXAMPLE 2 Igniter Fabrication

Powders of a conducting composition (30 vol % MoSi₂, 20 vol % SiC, remainder Al₂O₃) and an insulating composition (20 vol % SiC and 80 vol % Al₂O₃) were mixed with about 16 wt % water and 5 wt % Methyl Cellulose (Dow A4M) to form two pastes. The two pastes were loaded into the barrel of a piston extruder with the conducting paste forming a cylindrical core and the insulating paste forming a cylindrical sheath. The two mixes were co-extruded to form a coaxially clad rod of about 0.300″ diameter. The rod was then cured at 65° C. to remove the moisture and cut to 1-3″ lengths. The samples were dip-coated to coat one-half of the length with a slurry of conducting composition (30 vol % MoSi₂, 20 vol % SiC, remainder Al₂O₃) and the remaining half with a slurry of resistive composition (15 vol % MoSi₂, 20 vol % SiC, remainder Al₂O₃). The slurry contains dispersants and a low viscosity base fluid such as containing isopropyl alcohol, PEG 400, SANTICIZER 160, BUTVAR B76, 111M dispersant. The coated sample was presintered in Argon atmosphere at 1200° C. to bum out the binders, coated with boron nitride and densified at 1750° C. for 1 hour under a glass-hot isostatic press. The densified parts were cleaned up grit-blasting and an electrical circuit was formed by cutting ⅛″ from tip of the rod at the end coated with the conducting layer. The core and the outer surface at the cut end separated by the insulating layer form the two legs of the igniter and when connected to a power supply at a voltage of 60 volts attained a temperature of about 1200° C.

EXAMPLE 3 Further Igniter Fabrication

Admixed ceramic powders to form a resistive composition (15 vol % MoSi₂, 20 vol % SiC, remainder Al₂O₃) and an insulating composition (20 vol % SiC and 80 vol % Al₂O₃) were separately mixed with about 16 weight % water and about 5 weight percent methyl cellulose (available from Clariant) to form two ceramic pastes. The two pastes were loaded into the barrels of two-piston extruders assembled to feed into a two-layer coextrusion die. The insulator paste formed the core and the resistive paste formed a skin layer of about 0.015 inches thickness. The coextruded rod had an outer diameter of about 0.25 inches. The rod was then cured at about 65° C. to remove moisture and cut to 1.25 inches lengths. The samples were debinded and densified. The densified parts were cleaned by grit blasting and an electrical circuit formed by cutting grooves on opposite sides to ⅛ inches from the end.

The invention has been described in detail with reference to particular embodiments thereof. However, it will be appreciated that those skilled in the art, upon consideration of this disclosure, may make modification and improvements within the spirit and scope of the invention. 

1. A method for producing a resistive igniter, comprising extruding a ceramic element.
 2. The method of claim 1 wherein the ceramic element comprises two or more regions of differing resistivity.
 3. The method of claim 1 wherein the ceramic element comprises regions of differing resistivity through a cross-section of the element.
 4. The method of claim 1 further comprising removing at least a portion a region of a first resistivity to expose a region of a second, distinct resistivity.
 5. The method of claim 1 wherein the first region has a lower resistivity than the second region.
 6. The method of claim 1 further comprising applying one or more ceramic compositions to at least a portion of the ceramic element.
 7. The method of claim 6 wherein a conductive ceramic composition is applied to the ceramic element.
 8. The method of claim 6 wherein at least two distinct ceramic compositions having differing resistivities are applied to the ceramic element.
 9. The method of claim 1 further comprising densifying the extruded ceramic element.
 10. The method of claim 1 wherein a portion of the igniter interior is removed.
 11. The method of claim 1 wherein a ceramic tile element and the tile element is cut to provide a plurality of igniter elements.
 12. A ceramic igniter element obtainable by extruding a ceramic element.
 13. The ceramic igniter element of claim 12 wherein the element comprises two or more regions of differing resistivity.
 14. The igniter element of claim 12 wherein at least a portion of a region of a first resistivity has been exposed to expose a region of a second, distinct resistivity.
 15. The igniter element of claim 14 wherein the first region has a lower resistivity than the second region.
 16. The igniter element of claim 12 wherein one or more ceramic compositions are applied to at least a portion of the formed ceramic element.
 17. The igniter element of claim 12 wherein the igniter element has a substantially rounded cross-sectional shape for at least a portion of the igniter length.
 18. The igniter element of claim 12 wherein the igniter element has a non-circular cross-sectional shape.
 19. A method of igniting gaseous fuel, comprising applying an electric current across an igniter of claim
 12. 20. A method of claim 19 wherein the current has a nominal voltage of 6, 8, 10, 12, 24, 120, 220, 230 or 240 volts.
 21. A heating apparatus comprising an igniter of claim
 12. 